Is the weight on the aircraft always the same?

The masses are currently constant and set to about maximum landing weight for most airliners. Aircraft like the A319, A320 and A321 of course have different masses and their inertia and gravitational weight force is properly simulated. In fact different parts of the aircraft each have their own mass and move independently and are held together by flexible joint connections. With this physical effects don't need to be faked, they are a natural result of the computation (e.g. gyroscopic effect, oscillations, etc.). Likewise our plans are, if we come around to adding adjustable masses, then we also want realistic fuel consumption, fuel mass decrease and inertia changes (fuel distribution affecting the roll inertia for example or usable as trim-fuel in Concorde, ...). There are many changes that need to be made such as new menus, adding fuel systems, fully testing all kinds of center of gravity positions on all aircraft with different masses, load-sheets and assistance for fuel & load planning and weight and balance, fuel consumption after time-skip, etc. So this is not an easy change but an important one.